According to Genesis 13:18, after parting from Lot, "Abram moved his tent, and came and lived by the great tree of Mamre, which is in Hebron, and built an altar there to the Lord." (The Masoretic text always has the "great trees of Mamre," but the Septuagint consistently uses the singular. The credibility of the Septuagint has risen since the discovery of the Dead Sea Scrolls, and we shall follow it.)

In Genesis 18:1, Yahweh appears to Abraham:

Yahweh appeared to him by the great tree of Mamre, as he sat in the tent door in the heat of the day. He lifted up his eyes and looked, and saw that three men stood opposite him. When he saw them, he ran to meet them from the tent door, and bowed himself to the earth, and said, “My lord, if now I have found favor in your [singular in Hebrew] sight, please don’t go away from your servant. Now let a little water be fetched, wash your [plural] feet, and rest yourselves [plural] under the tree. I will get a morsel of bread so you can refresh your heart. After that you may go your way, now that you have come to your servant.”
The unique combination of singular and plural, the plural being three, was taken by Christians as First Testament evidence that the Holy Trinity visited Abraham. It established the special importance of Mamre for them. The account continues with the preparation of a meal for the guests (18:6-14):

They said, “Very well, do as you have said.” Abraham hurried into the tent to Sarah, and said, “Quickly prepare three measures of fine meal, knead it, and make cakes.” Abraham ran to the herd, and fetched a tender and good calf, and gave it to the servant. He hurried to dress it. He took butter, milk, and the calf which he had dressed, and set it before them. He stood by them under the tree, and they ate. They asked him, “Where is Sarah, your wife?” He said, “See, in the tent.” He said, “I will certainly return to you when the season comes round. Behold, Sarah your wife will have a son.” Sarah heard in the tent door, which was behind him. Now Abraham and Sarah were old, well advanced in age. Sarah had passed the age of childbearing. Sarah laughed within herself, saying, “After I have grown old will I have pleasure, my lord being old also?” Yahweh said to Abraham, “Why did Sarah laugh, saying, ‘Will I really bear a child, yet I am old?’ Is anything too hard for Yahweh? At the set time I will return to you, when the season comes round, and Sarah will have a son.”

The name of Isaac, the child of this promise, is yitzhak in Hebrew, meaning "He will laugh." Here we have part of the fulfillment of God's promise to Abraham, as stated earlier, for example in Genesis 15:5.

Yahweh brought him outside, and said, “Look now toward the sky, and count the stars, if you are able to count them.” He said to Abram, “So shall your seed be.”

But this promise harks farther back, to Genesis 12:1-3.

Now Yahweh said to Abram, “Get out of your country, and from your relatives, and from your father’s house, to the land that I will show you. I will make of you a great nation. I will bless you and make your name great. You will be a blessing. I will bless those who bless you, and I will curse him who curses you. All of the families of the earth will be blessed in you.”

This passage is the transition from the primeval world history (Genesis 1-11) to the particular history of Abraham and his descendants. In the former, a pattern is established (first perceived by von Rad ): God creates, human beings sin, God punishes them, but then he performs a redeeming act that enables humankind to continue. He makes clothes for Adam and Eve, He puts a mark on Cain's forehead, He saves Noah's family and the animals, two of each kind. Then comes the sin of the Tower of Babel. God destroys it and scatters the nations, confusing their tongues. What will be the new redeeming act? Precisely here we get the birth and call of Abram: All the (dispersed) families of the earth will be blessed by him, through his seed. That is, they will be brought back into their proper relationships with one another and with God. Significant parts of the First and Second Testaments are devoted to showing how this ultimate act of redemption is to unfold.

2. The cave.

Some years after giving birth to Isaac, Sarah died "in Kiryat Arba, that is Hebron." In the gate of the city, Abraham negotiated with Ephron the Hittite to purchase a cave in which to bury her. Ephron offered the cave as a gift, but Abraham insisted on buying it. Thus, although "a stranger and sojourner" (Genesis 23:4), he achieved a legal claim to a piece of the land God had promised him. The language has a distinctive legalistic ring (23:17-20):

So the field of Ephron, which was in Machpelah, which was before Mamre, the field, the cave which was in it, and all the trees that were in the field, that were in all of its borders, were deeded to Abraham for a possession in the presence of the children of Heth, before all who went in at the gate of his city. After this, Abraham buried Sarah his wife in the cave of the field of Machpelah before Mamre (that is, Hebron), in the land of Canaan. The field, and the cave that is in it, were deeded to Abraham for a possession of a burying place by the children of Heth.

We find the same phrasing each time one of the patriarchs is buried: the cave is in the field of Machpelah before Mamre. Since the mental map of Biblical humans generally had them facing east (as in the Hebrew of Zechariah 14:8, for example) the cave would have been east of Mamre.

Where then was Mamre?

Clearly, if the Bible is correct, Mamre must have been just west of the place where the patriarchs and matriarchs are buried. The burial place has been identified for at least 2000 years with a cave in Hebron, over which Herod built a magnificent temenos that still stands today.

Slightly more than one kilometer northwest of Hebron (the Hellenistic-Roman Hebron) is a site called khirbet nimra, the Ruin of Nimra, which has yielded up the remains of a large building from the 6th-5th centuries BC, but nothing earlier. Hebron itself was deserted at this time, the Persian period, although later it developed in the valley near the cave. Since "m" has a way of becoming "n," the name Nimra is tantalizingly close to Mamre.

Moreover, in the Josephus' Jewish War (4:533), after mentioning the patriarchs' tombs in the "little town" of Hebron, Josephus describes "an immense terebinth" of his own day, "said to be as old as creation." The sentence only makes sense, occurring where it does, with reference to the Biblical tradition of Mamre. Josephus locates the tree eight stadia from the town. That amounts to slightly more than one kilometer, the same distance as Nimra. (Jericke, pp. 48-52.) If this terebinth was at Nimra, that would be another reason to identify the latter with Mamre. The cave over which Herod built his structure would indeed be "before" it, southeast to be exact, at the end of a field. (For a view toward the south, go here.)

The Ruln Of Nimra

Strangely, in addition to the Hebron temenos, Herod (or someone copying him) builtanother structure (or part of it) two miles north of the ancient city, and the Byzantines identified it as Mamre. It was distinguished by a great tree that died about 1650 years ago.

(To add another puzzle, Josephus does not mention either of the Herodian buildings: not the one in Hebron and not the one two miles to the north. The Jewish sources do not mention them either.)

In the Crusader period, the Mamre tradition moved. An eminent tree at the foot of Hebron's tell was identified as the one under which Abraham had received his guests.

In the 19th century another tree became impressive. This stood about 1.5 miles northwest of the tell. Russia bought the property and built a monastery there, identifying it as the tree of Mamre. A remnant can be seen today.